An apparatus of the aforementioned type is described in the German Offenlegungsschrift No. 2632463. In this known apparatus, the paper feed mechanism is powered by the printing platen of the office machine via gear belts which drive the top sheet separating roller that pulls individual sheets from the stack of paper in the paper magazine. The apparatus described there also includes paper transport rollers which are also driven by the gear belts and serve to deliver imprinted paper from the printing platen to a paper holder. The apparatus described in the aforementioned publication thus does not require its own power source and driving motors for the purpose of top sheet transport, paper feed and paper removal. Furthermore, all the transport operations are controlled by the associated printing office machine so that the paper feed mechanism requires no independent control systems and it is not necessary to synchronize the operation of the office machine and the paper feed mechanism.
In the known apparatus, the direction of rotation of the top sheet separating rollers results in the same paper advance direction as that provided by the printing platen of the office machine but the separating roller turns with a somewhat lower tangential speed. As soon as the top sheet of paper has been pulled off by the separating roller from the stack of paper and has reached the entrance gap of the printing platen, the latter takes over the further transport at increased speed. Accordingly, the onset of the transport of the paper by the printing platen depends on a number of factors among which is the frictional effect of the separating roller and the transporting speed and duration from the stack of paper to the platen. However, the speed or duration of the paper transport is subject to fluctuations because when the paper is transported to the printing platen, it must pass over a braking roller which turns in the direction opposite to the direction of motion of the sheet. Accordingly, the moment at which the printing platen can begin the further transport of the sheet is not well defined and this fact results in fluctuations and imprecision of the line positioning of the sheet for the subsequent imprinting which is under automatic control.
The German Pat. No. 207093 discloses an apparatus for feeding single sheets of paper from a stack of paper to the printing platen of an office machine wherein the separating roller is powered by the printing platen via a power transmitting gear or wheel. The intermediate wheel is a friction wheel that lies on the printing platen subject to the weight of the paper feed mechanism. In the apparatus described here, the separating roller also rotates in the same direction as the printing platen, causing individual sheets of paper to be grasped immediately by the printing platen. The initial position of the paper for the purpose of beginning the transport by the printing platen is thus also poorly defined resulting in the aforementioned problem. U.S. Pat. No. 2,887,208 describes a paper feed mechanism in which the top sheet separating function is performed by a circulating belt which is driven by the printing platen via a gear belt. In this apparatus the paper transport is initiated by the separating belt when the printing platen rotates in the direction of forward transport. Accordingly, the above cited disadvantages of the imprecision of the initial position of the sheet when it enters the region of the printing platen are also present. This apparatus includes a manual clutch which prevents the corotation of the top sheet separating belt if the printing platen must be turned backward during the imprinting of the paper.